Did they say broncholitis? Poor little dd, and poor you. Its rough when they're so little.
My dd2 had bronchiolitis quite badly at 4 weeks old, and was in hospital for a week (she had other airway issues which meant she was very ill with it) with an 8 week old and a 3 month old....so I've seen a few very young ones with it, and recover (fast, once they get past the 5th day) from it.
The things to look out for are dehydration from not drinking, and any difficulty breathing - if her ribs pull in when she breathes, or if she's breathing faster than usual, for instance. I remember dd vomiting all her feeds back up, and having bubbly spit around her mouth - both can be signs of bronchiolitis.
Antibiotics wouldn't do anything if it IS bronch, because its usually caused by a virus - often RSV. But all the admitted kids I saw were on them anyway, in case of secondary chest infection, so probably a good thing your dd is on them regardless.
If the 4oz have stsayed down, thats good. If you have a humidifier to put in her room, thats great - if not, a wet towel or bowls of water on a hot radiator can help.
Hope the ABs make a difference, fast, and that she is better during the night...but don't hesitate to go back if she's isn't, IME hospitals are very very used to helping small babies cope with bronch and babies can get sick very fast, though they recover equally fast once they turn that corner.
Wishing you a peaceful night, xx